This invention relates to an improvement in forming wet-laid, non-woven fabrics from aqueous, fiber dispersions. This invention is particularly related to the formation of a stable, viscous, uniform, aqueous dispersion in which the individual fibers do not become entangled. This invention is quite particularly concerned with forming non-woven fabrics from relatively long and thin, flexible, synthetic, staple fibers, such as polyester fibers of 1/2 to 11/2 inches in length and 1.25 to 3.0 denier.
Various processes for forming non-woven fabrics by wet-laying synthetic fibers on paper making equipment are known in the art. Typically, in such processes, the fibers are laid on a forming wire or wire screen as either an aqueous dispersion or as an aqueous foam. See, for example, U.S. Pat. No. 3,808,095 and U.S. Pat. No. 3,839,142.
In all of the heretofore available processes for wet-laying a non-woven fabric, no substantial difficulties have been encountered in utilizing relatively thick and short, inflexible fibers, such as 1.5 denier by 1/4 inch fibers, 6.0 denier by 3/4 inch fibers, and 15.0 denier by 11/8 inches fibers. However, such processes have been unsatisfactory for forming non-woven fabrics from relatively long and thin, flexible synthetic fibers, such as 1.5 denier by 1 inch fibers and 3.0 denier by 11/2 inches fibers. The relatively long and thin, flexible, synthetic fibers have tended to become entangled when suspended in the aqueous dispersions or foams used for wet-laying the fibers on the forming wire. Such fibers, when entangled, have formed knits, bundles and strings in the resulting, non-woven fabrics. The presence of such knits, bundles and strings, in general, has rendered such fabrics commercially unacceptable.
Means have been sought therefore for uniformly dispersing long, thin, flexible, synthetic fibers so that the fibers cannot become entangled. Certain foam dispersions of the fibers have been useful for this purpose. See, for example, British Pat. No. 1,129,757, Canadian Pat. No. 787,649, and U.S. Pat. Nos. 3,716,449, 3,837,999 and 3,007,840. However, the use of foam dispersions has been somewhat limited. This is because such foams are rather difficult and expensive to handle and because the resulting fabrics have tended to be weak and, for this reason, rather difficult to handle. Thus, the use of liquid phase dispersions of fibers has been preferred.
However, severe difficulties have been encountered in the use of liquid phase, i.e., aqueous, dispersions of long, thin, flexible, synthetic fibers, particularly hydrophobic fibers.
Relatively long and thin, flexible, synthetic, staple length fibers generally have been very difficult to disperse in water. The resulting dispersions also have been hard to maintain and to transport to the forming wire as uniform dispersions. However, unless these fibers have been completely dispersed in the liquid medium and maintained in a completely dispersed condition, undesirable entangling and flocculating of the fibers, to produce knits, bundles and strings of the fibers, have occurred to a substantial extent.
These flexible fibers also have been especially prone to flocculate and to thereby form knits and bundles when being dispersed in water. The fibers have tended to bend, twist and curl and to touch other nearby fibers in the aqueous medium, particularly when the aqueous medium has been agitated or subjected to turbulence. When the fibers have been free to bend or touch other fibers, the inevitable result has been the formation of knits, bundles and other undesirable fiber entanglements, such as strings, in the resulting aqueous dispersions and in the resulting non-woven fabrics. This problem has been particularly aggravated with crimped fibers, the crimps of which act as entangling hooks and which readily produce, as a result, knits and long strings.
Further, the resulting dispersions generally have been hard to uniformly apply to the forming wire. This has been because the aqueous media utilized in the dispersions have tended to drain through the forming wire too quickly. In fact, the drainage rate from the aqueous dispersions has been so high that it had not been possible to use shake mechanisms, such as are common in the making of paper, for distributing the fibers more uniformly in the resulting webs.
Thus, means have been sought for expeditiously providing a uniform, water dispersion of flexible fibers, which is stable during periods of storage and of transport to the forming wire and which is adapted to provide a uniform fiber distribution when applied to the forming wire.
One means for promoting the dispersion of the flexible fibers in water and for maintaining the fiber dispersions has involved treating the fibers and/or the water with one or more chemical agents which promote the wetting of each fiber with water. With hydrophilic, synthetic fibers, such as viscose rayon, cellulose acetate and polyvinyl acetate, wetting the fibers has not been much of a problem. Hence, dispersing such fibers, little or no wetting agent has been required to disperse the fibers. On the other hand, wetting hydrophobic, synthetic fibers made from polymers such as polyamides, polyesters, polyolefins, phenolics and the like has been a more difficult problem since such fibers do not wet easily. Hence, relatively large quantities, e.g., about 0.1% by weight, of a wetting agent have been required in the liquid media to disperse such fibers.
However, since most wetting agents or dispersants are also good foam generating agents, particularly when present in amounts adequate to substantially wet hydrophobic fibers, the use of dispersants often has tended to create copious quantities of unwanted, surface foam, even under gentle agitation conditions. The surface foam produced has tended to float the fibers out of the dispersion. When defoaming agents have been added to dispersions of fibers, the fibers have tended to flocculate, thereby making the formation of a uniform web more difficult.
The use of dispersants which are not good foam generating agents also has been tried. See, for example, U.S. Pat. No. 3,067,087 and Canadian Pat. No. 921,210. With intense agitation and using such dispersants, relatively long and thin, flexible, synthetic fibers have been dispersed in water. However, the use of such dispersants has not in any way diminished the tendency of flexible fibers to become entangled when agitated in liquid media for more than a brief period or the tendency of such fibers to floccuate when removed from the region of high shear agitation, e.g., when being transported to the forming wire. Nor have such dispersants improved the drainage characteristics of the aqueous dispersions on the forming wire. Thus, the use of dispersing agents alone has not completely solved the problems associated with forming and wet-laying liquid phase dispersions of relatively long and thin, flexible, synthetic fibers.
In dispersing fibers, it has been observed that, when the viscosity of the liquid media is increased, fiber flocculation is reduced. For this reason, either with or without the use of dispersants, adding thickeners, such as natural and synthetic gums, to fiber and water mixtures has been tried. The use of thickeners for raising the viscosity of the water has been found useful for forming and maintaining dispersions of fibers. See, for example, Canadian Pat. No. 949,791 and U.S. Pat. Nos. 2,810,644, 3,013,936, 3,098,786, 3,794,557, 3,808,095 and 3,834,983. The use of thickeners also has been found to modify the drainage characteristics of water and fiber dispersions on the forming wire. See, in this regard, U.S. Pat. No. 3,391,057. However, even with such thickeners, dispersing relatively long and thin, flexible, synthetic fibers in liquid media, such as water, and maintaining the fibers in a dispersion, without forming knits, bundles and strings of the fibers, has continued to be a problem.
Another significant difficulty in forming non-woven fabrics from liquid phase dispersions has been in providing fabric webs which can be removed from the forming wire without tearing them or pulling them apart.
To increase the initial, wet web strength, in some instances, hydrated (fibrillated) wood or other natural fibers and/or fibrillated, synthetic fibers have been combined with non-fibrillated, synthetic fiber furnishes. Such combinations have tended to hold non-woven webs together while they have been transferred from a moving, forming wire, across unsupported draws, to wet presses or other treating equipment, where a binder has been added to hold the fibers together more permanently. In such webs, before the addition of any adhesive, the webs have been held together, in part, by the mechanical interlocking of the fibrillated fibers. However, the use of the fibrillated, natural or synthetic fibers as part of the furnish has not proven satisfactory for non-wovens intended for use as replacement fabrics for textiles. This has been because of the stiff, "papery" hand imparted by these fibrillated fibers to the resulting, non-woven fabrics.
Another technique for increasing the initial, wet web strength of non-fibrillated fibers has included coating or encapsulating the fibers with latex polymer binders. These binders have held the sheets together and allowed their continuous removal from the forming wire without their breaking or tearing. However, the use of latex polymer coatings, though providing fabrics of softer and more textile-like properties, has tended to be rather expensive. Such coatings have had the added disadvantage of being tacky, thus making it difficult to maintain clean and non-tacky machine conditions.
Still another technique for holding the wet webs together has involved the very careful control of the amount of water in the web as it is transferred from the forming wire. See, in this regard, U.S. Pat. No. 3,223,581. One disadvantage of such a process has been that its usefulness has been limited to fibers having essentially smooth, flat surfaces for providing large, area surface contact among the fibers forming the sheet. Round and other fibers having no flat surfaces have not worked with this technique. In addition, such fibers have produced relatively dense, stiff and "papery" sheets which are undesirable in non-wovens intended for textile uses.